The present invention relates to an image reader for a facsimile apparatus or like image processing apparatus and, more particularly, to an image reader which shortens a reading time and frees image signals from noise.
Image reading means installed in an image processing apparatus, such as a document reader of a facsimile apparatus, is implemented using a line image sensor which functions to decompose one line of images into pixels and then subject them to photoelectric conversion. Generally, a line image sensor comprises a light-sensitive section where a plurality of lightsensitive elements such as photodiodes are arranged in an array, and a signal selector section where output signals of the lightsensitive elements are sequentially selected. The basic construction of such a line image sensor is shown in FIG. 1.
In the line image sensor, generally 10, shown in FIG. 1, a capacitance Cd represents a coupling capacitance of a photodiode, or light-sensitive element, PD, while D.sub.L represents a capacitance developing in a wiring between the photodiode PD and a circuit to follow (e.g. amplifier). A resistor R is a current-limiting resistor. A switch SW comprises a MS (metal oxide semiconductor) switch or like semiconductor element. A voltage V.sub.D is applied to the line image sensor 10 from a power source, not shown.
Assume that the switch SW is turned on to charge the capacitances Cd and C.sub.L and then turned off to set up an image signal storing condition. In this condition, a photocurrent complementary to a quantity of received light, i.e., a pixel luminance associated with a read image, develops in the photodiode PD to discharge the capacitance Cd. When the switch SW is turned on again, the photodiode PD produces an output voltage Vout which based on charge conservation is expressed as: EQU Vout=V.sub.D -(Ip.multidot.T/(Cd+C.sub.L)) Eq. (1)
where T is the interval between consecutive turnons of the switch SW, or image information storing priod.
Meanwhile, in the case where the whole charge stored in the capacitance Cd is discharged by the photocurrent Ip which has flown during the storing tme T, the output voltage Vout of the photodiode, or saturation output Vsat, is produced by: EQU Vsat=C.sub.L .multidot.V.sub.D /(C.sub.L +Cd) Eq. (2)
Hence, the output Vout of the photodiode PD varies from the source voltage V.sub.D to Vsat complementarily to the photocurrent Ip which has flown over the storing time T, i.e. luminance of the associated pixel. In this manner, image signals corresponding to pixel densities are provided.
Assume a line image sensor which reads an A4 format document eight dots per millimeter and has a reading width of 216 millimeters. Such a line image sensor, therefore, comprises the above-mentioned photodiodes PD and switches SW in 1,728 pairs in total. Where this type of line image sensor is driven as a single element, the capacitance C.sub.L increases to a significant level. The Eq. (2) teaches that an increase in the capacitance C.sub.L is reflected by a decrease in the level of the saturation output Vsat which in turn narrows the available dynamic range.
An implementation heretofore employed to preserve a desired dynamic range consists in dividing the light-sensitive cells of a line image sensor into a plurality of blocks and driving the cells on a block-by-block basis. For example, 1,728 pairs of photodiodes PD and switches SW have been divided into twenty-seven blocks each comprising sixty-four pairs. An example of such a prior art image reader is shown in FIG. 2. In the image reader 20 shown in FIG. 2, photodiodes PD and switches SW are each divided into n blocks each comprising m photodiodes or switches, while outputs of the individual blocks BL1-Bln are applied to an amplifier AM via switches SL1-SLn. A controller (not shown) controls the switches SL1-SLn, SW1l-SWln and SWnl-SWnm as indicated by waveforms a to j in FIG. 3, whereby one complete line of picture signals Va are produced.
First, after the switch SL1 associated with the block BL1 has been turned on, the switches SW11-SW1m are sequentially turned on each for a charging period so as to apply output signals of the respective photodiodes PD to the amplifier AM. As the block GL1 is fully read, the switches SL1 is turnd off and, instead, the switch SW2 is turned on to read the next block BL2. Thereafter the same procedure is sequentially repeated on the other blocks down to BLn.
The problem with the prior art image reader of the type described is that noise NZ entailed by the turnon and turnoff of the blocks BL1-BLn, has great influence on image signals Va. It therefore has been commonly practiced to set up a delay td between a switching action of any of the switches SL1-SLn and the subsequent start of operation of the associated block BL, thereby presenting noise NZ from being introduced into the image signals. However, this is not at a cost, i.e., a longer time ncessary for the reader to read one line of pixels. As shown in FIG. 4, although a preamplifier AM.sub.1, AM.sub.2, . . . , or AMn may be interposed between each of the switches SL1-SLn and its asociated block BL-BLn in order to minimize the influence of noise NZ on the image signals Va, such is undesirable from the economy standpoint because the same number of preamplifiers as the blocks, n, would be required.
Meanwhile, in reading a signal out of each cell, it is necessary to preserve the "on" state of the associated switch SW for a certain priod of time (hereinafter referred to as a charging time) so that the capacitances Cd and C.sub.L may be charged, as previously stated. The charging time is determined by a time constant which is provided by multiplying a sum of the capacitances Cd and C.sub.L by a resistance value R. Usually, since the output width of a cell should be reduced beyond a certain limit, it is impossible to employ an excessively small resistance value R and, therefore, an excessively short charging time. the result is the need for a certain reading time per cell and, as a whole, a substantial period of time consumed in reading one line of image signals. While the reading time may be shortened by driving all the blocks at the same time, such cannot be implemented without provision of a parallel-to-serial converter and other converting means which convert outputs of the respective blocks into one line of time-serial signals, resulting in an intricate construction which adds to the space and cost. Another possible approach for a shorter reading time is improving the response characteristics of the cells. However, this approach is disadvantageous in that a decrease in the coupling capacitance of a photodiode is accompanied by a decrease in the saturation output Vout which inherently narrows the output width.